The principal characteristic of the separation, removal or concentration of oxygen from the air is that usually there is no cost for the starting material, which is air. The cost of the oxygen produced or removed, depends essentially upon the following factors.
(a) Costs of equipment necessary for separating or concentrating oxygen,
(b) Costs of energy necessary for operating the equipment,
(c) When purified oxygen is needed, the cost of the purification step has to be taken into account.
Another characteristic is that separation or concentration of oxygen can be achieved either by separating oxygen or by separating nitrogen from air as a starting material.
Taking into consideration the above-described factors, various economically advantageous processes have heretofore been proposed. These include, for example, the process in which the air is liquefied at low temperatures to separate oxygen or nitrogen making use of difference in the boiling point between liquid oxygen (−182.9° C.) and liquid nitrogen (−195.8° C.). The apparatus employed is suited for producing large amounts of oxygen and the production of most of the oxygen and nitrogen in the world is based on this procedure. One disadvantage of the process is that it requires large amounts of power. Another is that large-scale equipment is necessarily site specific and portability is very difficult. Another is that it takes hours for switching on and switching off the plant.
In another approach the membrane separation system has been employed for the separation of oxygen and nitrogen from air [U.S. Pat. No. 5,091,216 (1992) to Hayes et.al. U.S. Pat. No. 5,004,482 (1991) to Haas et.al, U.S. patent application 20020038602 (2002), to Katz; et al.]. The main drawbacks of this method is the thin polymeric films used in the separation process are too weak to withstand the high differential gas pressures required for the separation and the purity of the product gas is only around 50%.
In the prior art, adsorbent which are selective for nitrogen from its mixture with oxygen and argon have been reported [U.S. Pat. No. 4,481,018 (1984) to Coe et.al, U.S. Pat. No. 4,557,736 (1985) to Sircar et.al, U.S. Pat. No. 4,859,217 (1989) to Chao; Chien-Chung, U.S. Pat. No. 4,943,304 (1990) to Coe et.al, U.S. Pat. No. 4,964,889 (1990) to Chao; Chien-Chung, U.S. Pat. No. 5,114,440 (1992) to Reiss; Gerhard, U.S. Pat. No. 5,152,813 (1992) to Coe et.al, U.S. Pat. No. 5,174,979 (1992) to Chao; Chien-Chung et.al, U.S. Pat. No. 5,454,857 (1995) to Chao; Chien-Chung, U.S. Pat. No. 5,464,467 (1995) to Fitch et.al., U.S. Pat. No. 5,698,013 (1997) to Chao; Chien-Chung., U.S. Pat. No. 5,868,818 (1999) to Ogawa et.al., U.S. Pat. No. 6,030,916 (2000) to Choudary et.al., U.S. Pat. No. 6,231,644 to Jaine et al.,] wherein the zeolites of type A, faujasite, mordenite, clinoptilites, chabazite and monolith have been used. The efforts to enhance the adsorption capacity and selectivity have been reported by exchanging the extra framework cations with alkali and/or alkaline earth metal cations and increasing the number of extra framework cations of the zeolite structure by modifying the chemical composition. The adsorption selectivity for nitrogen has also been substantially enhanced by exchanging the zeolite with cations like lithium and/or calcium in some zeolite types. They have been employed in processes for the separation or concentration of oxygen by removing nitrogen selectively from the air. However, the molecular sieves of these types have an isotherm, which follows Langmuir adsorption isotherm. As a result, when the pressure reaches 1.5 atmospheres absolute (ata) the increase in the adsorptivity is not large compared with the increase in the pressure. Moreover, a very large amount of nitrogen must be separated since the molar ratio of N2/O2 in the air is 4. Therefore, the advantage achieved by enlargement of the apparatus to permit the use of high pressure is rather small. This limits the application of this process to small volume installations. The maximum attainable oxygen purity by adsorption processes is around 95%, with separation of 0.934-mole percent argon present in the air being a limiting factor to achieve 100% oxygen purity. These adsorbents are highly moisture sensitive and the adsorption capacity and selectivity will decay in the presence of moisture. The chromatographic separation of oxygen and argon is also possible by using these adsorbents.
U.S. Pat. No. 4,453,952 (1984) to Izmi et.al. discloses the manufacture of an oxygen selective adsorbent by substituting the Na cations of zeolite A with K and Fe (II). The adsorbent shows oxygen selectivity only at low temperature and its preparation requires multistage cation exchange, adding to the cost of preparation. Cation exchange is carried out at around 80° C. using aqueous salt solutions of metal ions to be exchanged. This results into higher energy requirement as well as generation of effluents during exchange process. Furthermore, potassium exchange in zeolite leads to lower thermal and hydrothermal stability of the adsorbent.
Carbon molecular sieves are effective for separating oxygen from nitrogen because the rate of adsorption of oxygen is higher than that of nitrogen. The difference in rates of adsorption is due to the difference in size of the oxygen and nitrogen molecules. Since the difference in size is quite small, approximately 0.2 A°, the pore structure of the carbon molecular sieve must be tightly controlled in order to effectively separate the two molecules. In order to improve the performance of carbon molecular sieves, various techniques have been used to modify pore size. The most common method is the deposit of carbon on carbon molecular sieves. For example, U.S. Pat. No. 3,979,330 to Munzner et.al discloses the preparation of carbon containing molecular sieves in which coke containing up to 5% volatile components is treated at 600° C.-900° C. in order to split off carbon from a hydrocarbon. The split-off carbon is deposited in the carbon framework of the coke to narrow the existing pores. U.S. Pat. Nos. 4,528,281; 4,540,678; 4,627,857 and 4,629,476 to Jr. Robert, S. F. disclose various preparations of carbon molecular sieves for use in separation of gases.
U.S. Pat. No. 4,742,040 to Ohsaki et.al. discloses a process for making a carbon molecular sieve having increased adsorption capacity and selectivity by pelletising powder coconut shell charcoal containing small amounts of coal tar as a binder, carbonising, washing in mineral acid solution to remove soluble ingredients, adding specified amounts of creosote or other aromatic compounds, heating at 950° C.-1000° C., and then cooling in an inert gas. U.S. Pat. No. 4,880,765 to Knoblauch et.al., discloses a process for producing carbon molecular sieves with uniform quality and good separating properties by treating a carbonaceous product with inert gas and steam in a vibrating oven and further treating it with benzene at high temperatures to thereby narrow existing pores. Preparation of carbon molecular sieve is a multistep process with utmost care at each stage to get totally reproducible carbon molecular sieve. Additionally, the process is very high temperature process, which results into higher cost of the adsorbent.
U.S. Pat. No. 5,081,097 to Sharma et.al., discloses copper modified carbon molecular sieves for selective removal of oxygen. The sieve is prepared by pyrolysis of a mixture of a copper-containing material and a polyfunctional alcohol to form a sorbent precursor. The sorbent precursor is then heated and reduced to produce a copper modified carbon molecular sieve. Pyrolysis is a high temperature process making the whole process of preparation of the adsorbent an energy intensive process.
Another process uses a transient metal-based organic complex capable of selectively absorbing oxygen [U.S. Pat. No. 4,477,418 (1984) to Mullhaupt Joseph et.al.; U.S. Pat. No. 5,126,466(1992) to Ramprasad et.al.; U.S. Pat. No. 5,141,725(1992) to Ramprasad et.al.; U.S. Pat. No. 5,294,418(1994) to Ramprasad et.al.; U.S. Pat. No. 5,945,079 (1999) to Mullhaupt Joseph et.al; U.S. patent application 20010003950 (2001), to Zhang, Delang et al.]. The absorption by these complexes is reversible with changes in temperature and pressure so that it is theoretically possible to achieve separation or concentration of oxygen by means of a temperature swing or a pressure swing cycle of the air.
However, in practice, severe deterioration of the organic complex occurs with repeated cycles of absorption and liberation of oxygen. Moreover, the organic complex itself is expensive. Therefore, the use of this process is limited to special situations. The main drawback of this process lies in air and moisture sensitivity of the metal complexes used which lowers the stability of the adsorbent produced. Additionally, the cost of the metal complexes used in preparation of the adsorbent is very high.
U.S. Pat. No. 6,087,289 (2000) to Choudary et al. discloses a process for the preparation of a zeolte-based adsorbent containing cerium cations for the selective adsorption of oxygen from a gas mixture. Cerium exchange into zeolite is carried out under reflux conditions using aqueous solution of cerium salt at around 80° C. for 4-8 hours and repeating the exchange process several times. The main drawbacks of this adsorbent lie in observation of oxygen selectivity only in the low-pressure region. Furthermore, adsorbent preparation is a multi-step ion exchange process, which also generates liquid effluent.
European Patent 0,218,403 to Greenbank discloses a dense gas pack of coarse and fine adsorbent particles wherein the size of the largest fine particles is less than one-third of the coarse particles and sixty percent of all particles are larger than sixty mesh. Although not specifically stated, it is evident from the examples that these percentages are by volume. This system is designed primarily for enhancing gas volume to be stored in a storage cylinder. It is mentioned, however, that it can be utilized for molecular sieves. There is nothing in this application, however, which would give insight into the fact that significantly enhanced PSA efficiency could be obtained by combining coarse and fine particles of kinetically-selective sieve material in a single bed. It has been found in accordance with the present invention that, within certain limits as will be defined, a mixture of coarse and fine kinetically selective sieve particles will unexpectedly give enhanced PSA performance.
In another approach chemical vapour deposition technique was used for controlling the pore opening size of the zeolites by the deposition of silicon alkoxide [M. Niwa et al., J C S Faraday Trans. I, 1984, 80, 3135-3145; M. Niwa et al., M. Niwa et al., J. Phys. Chem., 1986, 90, 6233-6237; Chemistry Letters, 1989, 441-442; M. Niwa et al., Ind. Eng. Chem. Res., 1991, 30, 38-42; D. Ohayon et al., Applied Catalysis A-General, 2001, 217, 241-251]. Chemical vapour deposition is carried out by taking a requisite quantity of zeolite in a glass reactor, which is thermally activated at 450° C. in situ under inert gas like nitrogen flow. The vapours of silicon alkoxide are continuously injected into inert gas stream, which carries the vapours to zeolite surface where alkoxide chemically reacts with silanol groups of the zeolite. Once the desired quantity of alkoxide is deposited on the zeolite, sample is heated to 550° C. in air for 4-6 hours after which it is brought down to ambient temperature and used for adsorption. The major disadvantages of this technique are (i) Chemical vapour deposition, which leads to non-uniform coating of alkoxide which in turn results in non-uniform pore mouth closure, (ii) The process has to be carried out at elevated temperature where the alkoxide gets vaporised, (iii) The deposition of the alkoxide requires to be done at a slow rate for better diffusion and (iv) This method is expensive and lack of a commercial level at higher scale will be difficult.
At present nitrogen and argon containing less than 10 ppm oxygen is produced by using a deoxo hybrid system in which the oxygen is removed by reducing it to water over a catalyst with hydrogen.